Liquid crystal display devices are display devices utilizing electrooptical changes of liquid crystal, and an attention has been drawn to their features such that the devices are small in size and light in weight, and power consumption is small. Accordingly, in recent years, they have undergone remarkable developments as display devices for various displays. Among them, an electric field effect type liquid crystal display device of twisted nematic type (TN type) is representative wherein nematic liquid crystal having a positive dielectric anisotropy, is used, liquid crystal molecules are aligned in parallel with substrates at the respective interfaces of a pair of mutually opposing electrode substrates, and the two substrates are combined so that the alignment directions of liquid crystal molecules will cross each other.
With such a TN type liquid crystal display device, it is important to align long axial directions of liquid crystal molecules uniformly in parallel with the substrate surface and to align liquid crystal molecules with a constant tilt alignment angle (hereinafter referred to as a tilt angle) to the substrate.
As typical methods for aligning liquid crystal molecules in such a manner, two methods have heretofore been known. The first method is a method wherein an inorganic substance such as silicon oxide is vapor-deposited from an oblique direction to the substrate to form an inorganic film on the substrate, so that the liquid crystal molecules will be aligned in the direction of vapor-deposition. This method is not industrially efficient, although constant alignment with a predetermined tilt angle can be obtained. The second method is a method wherein an organic coating film is formed on the substrate surface, and the film surface is rubbed in a predetermined direction with a cloth of e.g. nylon or polyester so that liquid crystal molecules are aligned in the rubbing direction. By this method, constant alignment can be obtained relatively easily, and this method is industrially most commonly employed. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be mentioned. However, polyimide is most commonly employed from the viewpoint of the chemical stability, thermal stability, etc. As typical examples of polyimide used for such liquid crystal alignment films, those disclosed in JP-A-61-47932 may be mentioned.
The treating method for liquid crystal alignment by rubbing polyimide is an industrially useful method which is simple and excellent in productivity. However, as the demands for high precision and high performance of liquid crystal display devices have increased and new display systems corresponding to such demands have been developed, various problems of the rubbing method have been pointed out. For example, a STN (Super Twisted Nematic) system wherein the twist angle of a TN type liquid crystal display is increased, an AM (Active Matrix) system wherein switching elements are formed for individual electrodes, and a FLC (ferroelectric) or AFLC (antiferroelectric) system wherein ferroelectric liquid crystal or antiferroelectric liquid crystal is employed, may be mentioned as such examples. In the STN system, contrast is high, and scratches on the alignment film surface formed by rubbing, become display defects. In the AM system, the mechanical force or static electricity due to rubbing is likely to destroy the switching elements, and dusting by rubbing tends to lead to display defects. In the FLC or AFLC system, it is difficult to satisfy both high speed response and uniform alignment of smectic liquid crystal only by simple rubbing treatment. Thus, various problems of the rubbing method have been made clear.
For the purpose of solving such problems, a so-called "rubbingless" alignment method wherein liquid crystal is aligned without rubbing, has been studied, and various methods have been proposed. For example, a method wherein photochromic molecules are introduced to the alignment film surface, so that molecules on the alignment film surface are aligned by light (JP-A-4-2844), a method wherein molecular chains constituting an alignment film, are aligned by means of a LB film (Langmuir-Blodgett film) (S. Kobayashi et al., Jpn. J. Appl. Phys., 27,475 (1988)) and a method wherein an alignment film is press-bonded on a preliminarily alignment-treated substrate to transfer the alignment (JP-A-6-43458), have been studied. However, when the industrial productivity is taken into account, they can not be said to be useful as substitutes for the rubbing method.
Whereas, various methods have been proposed wherein periodical irregularities are artificially formed on the alignment film surface, and liquid crystal molecules are aligned along such irregularities. The most simple method of this type is a method wherein a replica having periodical irregularities is preliminarily prepared, and a thermoplastic film is heated and pressed thereon to transfer the irregularities onto the film (JP-A-4-172320, JP-A-4-296820, JP-A-4-311926 etc.). By this method, it is certainly possible to prepare a film having periodical irregularities on its surface efficiently, but it has been impossible to obtain practical reliability as high as a polyimide film used in the rubbing method. Whereas, a method has been proposed in which a light with high energy, such as electron rays (JP-A-4-97130), .alpha.-rays (JP-A-2-19836), X-rays (JP-A-2-2515) or eximer laser (JP-A-5-53513), is applied to a polyimide film having high reliability to form periodical irregularities on the film surface. However, to use a light source for such high energy, can not hardly be said to be an efficient treating method for alignment when an industrial production where alignment treatment is continuously carried out uniformly over the entire surface of a large size substrate, is taken into consideration.
On the other hand, as an efficient method for forming periodical irregularities on the surface of a polyimide film having high reliability, a photolithographic method may be mentioned. The polyimide is used as an insulating film for semiconductors by virtue of its high insulating property and excellent electric characteristics, and in recent years, a so-called photosensitive polyimide has been developed which is a polyimide having a photocurable nature by itself. There has been an attempt to form a periodical irregularities by a photolithographic method using this photocurable polyimide. By this method, irregularities have certainly been formed on the surface of the polyimide film, but since the photocurable polyimide was initially developed as an insulating film, the properties to align liquid crystal have been inadequate. Further, it has been necessary to apply a buffer coating layer (JP-A-4-245224), and consequently, the process has been complex and can not be regarded as an efficient treating method for alignment which can be a substitute for the rubbing method when the industrial productivity is taken into consideration.
As a new treating method for alignment which has recently been found, a method has been proposed in which polarized ultraviolet rays, etc. are applied to a polymer film surface to align liquid crystal molecules without carrying out rubbing treatment. The following reports are available as such examples.
W. M. Gibbons et al., Nature, 351, 49 (1991), Y. Kawanishi et al., Mol. Cryst. Liq. Cryst., 218, 153 (1992), M. Shadt et al., Jpn. J. Appl. Phys. 31, 2155 (1992), and Y. Iimura et al., Jpn. J. Appl. Phys. 32, L93 (1993).
These methods are characterized in that liquid crystal is aligned in a predetermined direction by irradiation of polarized light without requiring conventional rubbing treatment. These methods have merits such that they are free from problems such as static electricity and scratches on the film surface by the rubbing method, and the process is simple when industrial production is taken into consideration.
Namely, the liquid crystal alignment method using irradiation of polarized light proposed here is considered to be an attractive method as a new treatment method for liquid crystal alignment without requiring rubbing treatment, although it is still in a fundamental research stage.
As polymer materials used in the reports heretofore, specific polymer materials such as polyimides having polyvinyl cinnamate or an azo type dye mainly dispersed, have been employed from the necessity to obtain a photochemical sensitivity to polarized light, and it is disclosed that by applying polarized light on the surface of such polymer films, the liquid crystal molecules can be aligned in a predetermined direction.
However, in a case where liquid crystal alignment employing such irradiation of polarized light is to be practically applied in future, not only a function of liquid crystal alignment, but also various functions as a liquid crystal alignment film are simultaneously required to accomplish a high level of liquid crystal display. This means that the polymer material to be used as a liquid crystal alignment film is not simply limited to a specific material, and selection of a wider range of chemical structures, becomes important.
Further, from the viewpoint of the reliability and the alignment stability of liquid crystal molecules, it is considered preferred to use polyimide which has heretofore been used.
Namely, the object of the present invention is to use a more uniform highly reliable polyimide resin when liquid crystal alignment by irradiation of polarized light is applied to a practical liquid crystal display device, and to provide a treating method for alignment wherein a polyimide material type having a wide structural range for selection is used.